Cellulose esters, including cellulose acetate have been formed into semi-permeable hollow fibers and used as separatory membranes in a variety of processes including desalinization of sea water, ultrafiltration of aqueous and non-aqueous solutions, ion exchange processes, concentration of salts, purifying waste streams and the like. Permeable separatory membranes prepared from film-forming cellulose esters are disclosed in many U.S. Patents, the most pertinent of which that are known to applicants are Nos. 3,532,527 and 3,494,780. U.S. Pat. Nos. 3,532,527 and 3,494,780 describe a process of melt spinning cellulose esters, particularly cellulose triacetate and cellulose acetate, from a melt-spin composition consisting of a compatible plasticizer of the tetramethylene sulfone type, such as those disclosed in U.S. Pat. Nos. 2,219,006, 2,451,299 and 3,423,491 and a polyol having a molecular weight from about 62 to about 20,000; the weight ratio of the sulfolane plasticizer to polyol in the mixture is disclosed to vary from about 0.66:1 to about 5:1 and preferably from about 0.8:1 to 1.3:1. The stated purpose of varying the relative proportions of these materials was to modify the ability of the fibers to separate salt from sea water. Such fibers made by the processes of U.S. Pat. Nos. 3,532,527 and 3,494,780, while useful in the desalinization of sea water, are not satisfactory for use in hemodialysis as hollow fibers in artificial kidneys.
Cellulose acetate membranes having diverse forms has been the subject of extensive research funded by the National Institutes of Health and the Office of Saline Water since about the middle 1960's. The National Institute of Arthritis and Metabolic Diseases has also funded research directed to the modification of known cellulose acetate hollow fibers to evaluate their potential for use in artificial kidneys. A three year project of this type, having as its major objective the development of a cellulose acetate hollow fiber artificial kidney, was conducted by The Dow Chemical Company, Western Division Research Laboratories in 1971-1973, under NIH Contract No. 70-2302. Under that contract cellulose acetate fibers were made by melt spinning a mixture of cellulose acetate and triethylene glycol, and some of the resultant fibers were incorporated into artificial kidneys and clinically tested in hemodialysis. The best artificial kidneys which were made during that project, while successful in the sense that they were used safely in dialyzing a number of test patients in a clinic were nevertheless unsuccessful in that their concurrent transport properties for removal of water and low molecular weight solutes such as urea and creatinine were not as good as artificial kidneys then available which employed cellulose hollow fibers; the problem with these kidneys was that water removal rates were too high and the ratio of blood solute to water removed was too low, and the project was dropped.
Since the early 1970's, when hollow fiber artificial kidneys were first commercially made available by Cordis Dow Corp. in the United States, the hollow fibers used in such commercial artificial kidneys have been substantially exclusively cellulose fibers. These fibers have been either the product of the cuproammonium process or the process of Lipps U.S. Pat. No. 3,546,209. Although cellulose hollow fibers have enjoyed widespread market acceptance as the best form of semipermeable membrane for use in artificial kidneys to the present time, it is acknowledged by the skilled artisan that there are numerous, recurring production problems in melt spinning such fibers and incorporating them into leak-free artificial kidneys. For example, tensile strength of the fibers is relatively low and fiber breakage makes handling during fiber processing and assembly into a dialysis chamber both complex and difficult. Because of such difficulties with cellulose capillary fibers there is a continuing need for semipermeable capillary fibers which are inexpensive, easy to melt spin and process into artificial kidneys on a commercial scale, and which possess the capacity to remove blood solutes such as urea, creatinine, uric acid, and water at rates which are higher than those which characterize present day cellulose capillary fibers.
The primary objective of this invention is to provide a new cellulose acetate hollow fiber which is improved relative to heretofore known cellulose ester and cellulose hollow fibers in having selectively controllable permeability characteristics that make possible the fabrication of artificial kidneys containing such fibers which provide water and solute clearances that are superior to those which characterize present day commercial artificial kidneys containing cellulose hollow fibers. A related objective is to provide a process for making the improved cellulose acetate fibers of this invention.